Chronic pain is the most prevalent health problem worldwide and is often inadequately treated. Spinal cord stimulation (SCS) is a widely used, non-destructive, reversible therapy for treatment of chronic pain, particularly used when other conservative modalities have failed. Recently, the use of kilohertz frequency alternating current (KHFAC) SCS has shown potential for significantly improved outcomes, but conflicting results have been reported. KHFAC has been shown to produce a rapid and reversible conduction block in peripheral nerves. Preliminary evidence suggests that KHFAC SCS may produce pain relief without concomitant paresthesia (unlike conventional SCS), and thus significant excitement has been generated regarding this new modality. However, the mechanism by which KHFAC SCS might produce pain relief, and the differences between the neural response at kilohertz frequencies compared to conventional SCS, are poorly understood. We hypothesize that KHFAC SCS, as currently implemented, does not produce a block of neural activity but rather acts to modulate ongoing neural activity. The critical factors that determine the action of KHFAC SCS begin with the details of the technology itself, extend through its action on the spinal cord, and ultimately to the conscious perception of pain. Through the aims in this study, we anticipate demonstrating the mechanisms underlying the early clinical success (and failures) of KHFAC SCS, thus providing critical information for the field of neuromodulation. We further hypothesize that the unique features of KHFAC, which include the capacity to activate, block, and modulate activity in neural structures, could be fully exploited by the development of novel KHFAC SCS systems designed specifically to take advantage of these features. Such a system could have a significant impact on SCS clinical outcomes and responder rates. KHFAC SCS may represent one of the most exciting advances in the field SCS over its 40 year history. If successful, this project will provide the foundation for a new an powerful modality for the treatment of refractory chronic pain.